Polenta question....please help!

I was thinking of making a polenta dish for Christmas (needed a vegetarian and wheat free dish.....that would go nicely w/Beef Tenderloin). I make it with garlic, shallots, parm cheese, rosemary, etc, and then spread it in a dish to set. I cut it in squares and then sautee and crisp up in a bit of olive oil and butter, then top w/caramelized onions, mushrooms and rosemary. It's pretty tasty, but I'm dealing with a dilemma:

Can I do everything, including the sauteeing part, ahead of time, and then put on a sheet pan in the oven to "reheat" right before I put it out for dinner? I'm having about 20 people over and don't want to be standing over a pan last minute.

If this isn't possible.....I may have to change my dish!!! Maybe do a root veggie gratin? Thoughts?